If you’ve already created the plot, then you can use the bokeh.plotting.curplot() function to return the “current” plot, and then set its height and width attributes. If you are building up a Plot object using the lower-level interfaces (e.g. the examples in bokeh/examples/glyph/, then you can just set those attributes directly as well on the plot object or in the Plot() constructor.
Alternatively, if you are using any of the glyph generation functions in bokeh.plotting, you can pass the plot_width and plot_height keyword arguments, e.g.:
line(x,y, color="#0000FF", tools="pan,wheel_zoom,box_zoom,reset",
name="line_example", plot_width=800, plot_height=300)